Adverse rotorcraft-pilot couplings: modelling and prediction of rigid body RPC. Sketches from the work of European Project ARISTOTEL 2010 - 2013

Unfavourable Aircraft/Rotorcraft Pilot Couplings (A/RPCs), usually called pilot induced oscillations (PIO), manifested themselves since the early days of manned flight and may still create problems in modern configurations. In Europe, the ARISTOTEL project (Aircraft and Rotorcraft Pilot Couplings - Tools and Techniques for Alleviation and Detection) was set up with the aim of understanding and improving the available tools used to unmask A/RPCs. The goal of the present paper is to give an overview of the work performed on rotorcraft rigid body RPC. Rigid body RPC involve adverse coupling phenomena dominated by helicopter lower frequency dynamics with pilot in the loop. Using as example the Bo-105 helicopter enhanced by a rate command attitude hold control system, the paper will demonstrate the applicability of bandwidth-phase delay and OLOP criteria to unmask Cat I PIO and respectively Cat II PIO. The paper will introduce a novel on-line prediction algorithm, the so- called PRE-PAC (phase aggression criterion) based on analysis of the phase distortion between the pilot input and vehicle response. Special attention will be given to pilot modelling for RPC detection in the so-called boundary avoidance tracking (BAT) concept. In this sense, the paper will determine the critical boundary size leading to a RPC in a tracking task and will connect this to the optical tau theory. Bifurcation theory will be applied to a BAT pilot-vehicle system in a roll step manoeuvre mainly for prediction of Cat III PIO.